Thermal transmittance measurements of niobium at cryogenic temperatures

The first $O$(100s nm) of the inner surface Niobium-based superconducting RF (SRF) cavities are crucial to achieve high accelerating fields and a low surface resistance. Recent treatments aim to improve superconducting properties by tailoring the interstitial atom concentration or by depositing thin superconducting films. Yet, no investigation of the thermal characteristics of those surfaces after such treatments has been done, although this is a crucial property to cool the induced RF losses away and to maintain the superconducting state while high surface magnetic fields are applied. In this contribution, a newly developed experimental set-up is described, which allowed the first ever measurement of the thermal transmittance of Mid-T heat treated and Superconductor-Insulator-Superconductor (SIS) coated niobium samples. The results show that the SIS samples perform the same as standard niobium and that Mid-T heat treated samples have an improved thermal transmittance of 30%.